We investigated the distribution, diurnal variation, and time of expression of aromatic L-amino acid decarboxylase (AADC), a second-step enzyme of the monoamine synthetic pathway, in the rat suprachiasmatic nucleus (SCN), by immunocytochemistry and in situ hybridization. AADC immunoreactive and mRNA expressing cells were observed in the dorsomedial and dorsolateral portions of the SCN. Double labeling immunofluorescence demonstrated that about 70% of AADC neuron in the dorsomedial portion contained vasopressin immunoreactivity. AADC mRNA expression in the SCN showed day-night variation, with higher signals in the daytime than at night. Developmental study demonstrated that AADC immunoreactivity and mRNA in the SCN were expressed at embryonic day 18, the onset time for fetal circadian rhythm. These findings suggest that AADC neurons in the SCN may play some role in the generation and entrainment of circadian rhythm.